The objective of these studies is to analyze the early steps of T cell development in order to search for possibilities to increase T cell production in the thymus. Monoclonal antibodies are generated against immature T cell blasts. One antibody had a co-stimulatory effect on the proliferation of immature T cells and bone marrow cells, and recognized dipeptidyl peptidase DDPIV, a membrane associated proteolytic enzyme. The mechanism of the antibodies action was also investigated, and the results suggest the existence of two independent mechanisms; one that involves changes in enzyme activity and another that is associated with a signaling pathway. The identification of DDPIV as a hematopoietic and T-lineage activator opens new possibilities in the analysis of immature T cell proliferation. Another antibody (10.2), recognized a determinant on bone marrow cells that had the potential to proliferate in vitro and on precursors involved in CFU-spleen formation in vivo. These data provide direct evidence for a close functional relationship between an early hematopoietic precursor and the intrathymic T cell precursor and indicate that the 10.2 marker is important for colony formation. Switching T cell development towards the gamma/delta lineage would be beneficial in HIV infected young patients, since these cells lack CD4, the HIV receptor. In order to achieve this goal experiments were initiated that analyze the regulation of TcR gamma chain expression and rearrangement. Another possible way to modulate T cell development is to regulate the thymic colonization process. We analyzed this problem by developing an assay which measures the capacity of CD4-/CD8- hybridomas to invade spaces available under anchorage-dependent cells. The results demonstrate that the invasion process is fast and energy-dependent. The invasivity is regulated by secreted and cell-associated factors.